Backpacks are often used to carry various objects in a convenient manner. Oftentimes the objects may be heavy, such as textbooks or notebooks. The weight of the backpack and its contents is usually distributed through one or two shoulder straps; in the case of two straps, one for each shoulder of the person carrying the backpack. Wide shoulder straps help distribute the weight over the surface area of the carrier's shoulders, but may impede airflow to the shoulders.